In general, electric vehicles or hybrid electric vehicles make use of electric energy stored in a battery as an energy source. As the battery for such vehicles, a lithium-ion polymer battery is mainly used, and is being intensively studied.
In the case of gasoline vehicles, their engines are driven using gasoline as a fuel, and thus they do not have big difficulty in measuring a quantity of fuel. However, in the case of the electric vehicles or hybrid electric vehicles, because the battery is used as their power sources, it is difficult to measure residual energy accumulated in the battery.
Meanwhile, information on how much the current energy is left and thus how far the current energy can cover in the future is very important to drivers of the electric vehicles or hybrid electric vehicles.
Specifically, because the electric vehicles or hybrid electric vehicles are driven by the energy charged in their batteries, it is very important to check a state of charge (SOC) of the battery. Thus, there are developed various techniques, which are intended to check the SOC of the battery during driving to notify information on, for instance, a drivable distance to the driver.
Further, there are made various attempts to appropriately set an initial value of the SOC of the battery prior to the driving of the vehicle.
At this time, the initial value of the SOC is usually set with reference to open circuit voltage (OCV). This method is premised that the OCV is not varied according to an environment and serves as an absolute reference value of the SOC. However, according to various tests and papers, it turns out that the OCV is varied depending on temperature and time, rather than not varied according to an environment.
Meanwhile, in addition to the SOC as mentioned above, as an index for indicating the state of the battery for the electric vehicle, a concept of state of health (SOH) is used. More specifically, the battery has different property with the lapse of time. That is, an aging effect takes place. It is the SOH that is to detect a current state of the battery by predicting an aging degree in advance. Currently, unlike the estimation of the SOC, there is no definition of the SOH, as well as its accurate estimation method. Thus, there are proposed various methods alone.
For example, in order to measure factors indicating the SOH, necessary tests are carried out at laboratories or maintenance shops for measuring the SOC of the battery and other factors prior to mounting the battery to the vehicle or any other system. However, this approach takes much time, and thus only voltage of the mounted battery is measured in order to reduce the time. In this manner, the measurement of only the voltage of the battery is estimated that it is not considered to indicate a genuine state of load treatment capacity under various operation conditions required to perform the measurement in order to ensure reliable operation of the battery under the SOC of the battery or in the vehicle or other system to which the battery is mounted.